Lift systems currently have so-termed double doors, i.e. not only shaft doors, but also cage doors arranged at the lift cage. The opening and closing of the shaft doors is usually induced by the cage or the cage doors. For the safety of the users of the lift systems and the visitors in the buildings incorporating the lift systems it is of great importance for the respective setting of the shaft and cage doors to be coordinated with the position of the lift cage, i.e. the shaft and cage doors may open only when the lift cage stops at one of the provided boarding and disembarking stations, i.e. at the level of a story. For this purpose, the positions not only of the shaft doors, but also of the cage doors are monitored.
The shaft doors can usually be locked in their closed setting with the help of mechanical locking devices. Conventional monitoring systems monitor the setting of the shaft doors with the assistance of safety contacts; these safety contacts detect whether the mechanical locking devices adopt their locking setting or their unlocking setting. The safety contacts are closed when the locking devices are disposed in their locking setting and the shaft doors are closed. The safety contacts are integrated in a safety circuit, which in turn is closed only when safety contacts are closed. The safety circuit is so connected with the drive of the lift system that the lift cage in normal operation can be moved upwards or downwards only when the safety circuit is closed. If a shaft door is open and its locking device is in the unlocking setting, then the corresponding safety contact and thus the safety circuit are open, which has the consequence that the lift cage cannot perform any upward or downward movement except with the help of a special control or if service personnel bridge over the interrupted safety circuit.
Every lift system with such a conventional monitoring means has various disadvantages which are described in more detail in the following.                A safety circuit is in every case subject to inherent problems; including the length of the connections, the voltage drop in the safety circuit and the comparatively high assembly cost.        Despite the presence of a monitoring system with a safety circuit, unsafe or risky situations cannot be avoided. On the one hand, the safety contacts can be readily easily bridged over individually or in common, which is virtually equivalent to absence of the safety precautions. On the other hand, an open shaft door may indeed prevent movement of the cage, but if the cage is not disposed at the open shaft door the risk accordingly exists of falling through the open shaft door.        Intelligent or situation-appropriate reactions, for example when the safety circuit is open, are not possible, since the cage in every case is stationary; in particular, it cannot be avoided that persons are unintentionally trapped in the lift cage.        The monitoring system does not allow a specific diagnosis, i.e. when the safety circuit is open it can only be established that at least one safety contact and thus at least one locking device or at least one shaft door is open. However, it cannot be established which safety contact or contacts is or are open.        Precautionary maintenance is not possible, since there are no indications about the state of the safety contacts; it is thus not possible to service the lift system in advance and replace worn safety contacts in good time, but still at a point in time in which the lift system can be shut down without problems, except within the scope of a periodic inspection, wherein, however, in many cases taking the lift system out of operation—which is not necessary per se—is carried out. The availability of the lift is restricted, since an open safety contact always has the consequence of taking the lift system out of operation, even when another solution, for example not travelling in the affected shaft section, would be possible.        
A functionally improved solution can be achieved if a data bus is used for detection or transfer of the data which concerns safety, in conjunction with the setting of the shaft doors. Since, however, the corresponding data are safety-relevant, a safety bus has to be used. Such a safety bus and, in particular, the safety bus nodes required for that purpose are, however, comparatively expensive and therefore hardly come into consideration for standardized lift systems.
The object of the invention is thus to create an improved lift system of the kind stated in the introduction that with respect to safety precautions in conjunction with the setting of the shaft doors on the one hand avoids the disadvantages of the state of the art and on the other hand is comparatively economical.